This invention relates to dried fruit and vegetable products which also have a puffed consistency. This invention further relates to a process for making fruit and vegetable dried products which have a puffed consistency, are crispy but yet tender, and which further are not susceptible to browning or other discoloration during processing or thereafter.
Various techniques have been used over the years to produce dried fruits and vegetables. One such technique is air drying. In such techniques the fruits and vegetables are washed and then laid out exposed to air and sun to dry. After being dried to the ambient moisture level the dried fruits and vegetables can be contacted with antimicrobial and preservative agents and then packaged. In another technique the fruit or vegetable is dried by being exposed to heated currents of air. In such processes the fruits and vegetables would be washed and then placed in a chamber whereby a heated air at a set relative humidity is passed through the chamber at a given flow rate. Depending on the temperature and the humidity of the heated air the fruits and vegetables can be dried to a moisture level of as low as about 10 percent by weight of moisture. However such processes do not produce a puffed dried product. Rather, what is produced in most instances is a product that due to shrinkage is hard and chewy. In addition such products have a rough rather than a smooth texture when being consumed.
Such drying processes that are conducted at atmospheric pressure also have a limitation with regard to how much moisture they can remove from the fruits and vegetables. Generally, such processes can reduce the moisture level of fruits and vegetables down to about 10 percent by weight to about 45 percent by weight moisture. This limitation is due to many factors. One factor is that during the dehydrating process it is desired to keep the temperature of the fruits and vegetables below about 70.degree. C. If the fruits and vegetables are maintained at above about 70.degree. C. for any extended periods of time the sugars in the fruits and vegetables invert and change thereby changing the flavor of the fruits and vegetables undergoing drying.
It has been recognized in the art that in order to achieve very low moisture levels for dried fruits and vegetables that vacuum techniques would have to be used. In vacuum drying techniques the fruits and vegetables can first be air dried or dehydrated to a reduced level using a heated air stream. At this point the dried fruits and vegetables have a moisture level of about 10 percent by weight to about 45 percent by weight. The dehydrated fruit can then be vacuum fried or vacuum dried. Additionally while the fruits and vegetables are being exposed to the vacuum conditions heat can be applied to drive more moisture from the fruits and vegetables that are being dried. After being dried to the desired level the fruits and vegetables can then be packaged.
In vacuum frying the fruit or vegetable is placed in an edible oil and a vacuum drawn while the oil is heated. The hot oil serves as a medium to heat and remove the water in the fruits or vegetables. Vacuum frying techniques can reduce the water content of a fruit or vegetable to less than about 5 percent by weight. The end product is a hard, crispy piece of fruit or vegetable. However, this product will contain a residual amount of oil that can be up to about 30 percent by weight of the fruit or vegetable.
In order to further increase the drying speed of the vacuum processes the items to be dried can be cut into smaller pieces, or in the alternative macerated to a puree. This provides for increased drying speed since there is an increased surface area to volume. A dehydrated fruit or vegetable that has undergone at least part of the dehydrating treatment under vacuum conditions can also be produced in a puffed form. One method to induce puffing of the fruit or vegetable is to have a step of rapidly reducing the pressure on the fruit or vegetable to cause the contained water to rapidly vaporize and create a puffed product as it escapes from the fruit or vegetable. Since these fruits and vegetables are usually dried in an oil the pores of these puffed fruits and vegetables are impregnated with the oil.
In addition, regardless of the type of vacuum drying that is used, there remains the problem that there is a discoloration of the fruits and vegetables during this processing. This discoloration is usually a browning of the surface of the fruits and vegetables. It is believed that this browning is the result of two different natural processes that are occurring. One is enzymatic browning. For instance, polyphenoloxidase causes browning in apples and peaches. The other is non-enzymatic browning resulting from the usually high level of fructose and other monosaccharides in these fruits and vegetables. Browning also affects the nutritional and flavor properties of the dehydrated fruits and vegetables. In addition, it does result in a very unattractive color for the fruits and vegetables and would lead one to believe that the dehydrated fruits and vegetables are spoiled, or in the alternative that the dehydrated fruits and vegetables were made from spoiled fruits and vegetables. The net result is that the consumer would not want to purchase and consume this type of a product. Therefore, it is a primary objective of this invention to set forth a process for producing puffed dried fruits and vegetables which retain their natural color. It is a further object of the present invention to set forth a technique whereby sliced fruits and vegetables can be preserved from undergoing enzymatic browning or non-enzymatic browning prior to or after undergoing subsequent processing to form the puffed products and during storage.
The present discoveries are directed to a technique for making puffed fruits and vegetables that retain the color and taste of the original fruit or vegetable. The puffed fruit or vegetable is crispy, but yet tender and has a smooth texture when being consumed. In addition it does not contain any oil or equivalent substances since it is not produced by a vacuum frying technique. Rather it is produced through a vacuum drying step to produce the puffed consistency of the product. The vacuum drying can be preceded by an air drying. In addition the fruit or vegetable will undergo an infusion solution treatment to prevent discoloration. Further the product can be tenderized and the amount of puffing in the final fruit or vegetable product increased through a freeze-thaw step prior to the vacuum drying of the fruit or vegetable to tenderize. The net result is the ability to produce a crispy, puffed fruit or vegetable snack product that has the original fruit or vegetable color and which does not contain any oils within the pores of the fruit or vegetable.
Various U.S. patents disclose techniques for drying fruits and vegetables using vacuum drying and vacuum frying techniques. In U.S. Pat. No. 1,543,948 wafers or cakes of a fruit are introduced into a vacuum drying apparatus where the temperature is maintained at 70.degree. C. and the drying continued at a reduced pressure until the desired moisture level is reached. Usually the drying requires from six to eight hours. After vacuum drying the dried wafers or cakes are packed in a paper carton to prevent the loss of crispness through the adsorption of moisture from the atmosphere. Figs, raisins, and prunes can be dried using this technique.
U.S. Pat. No. 2,023,536 discloses a process for the vacuum drying of fruit. In this process a dehydrated fruit which can be in the form of a cake is placed within a vacuum chamber and a vacuum drawn. In addition, heat is applied to the vacuum chamber in order to raise the temperature of the fruit up to about 72.degree. C. After being dried, it can be further treated or can be packaged. The dried product is substantially crisp and dry.
U.S. Pat. No. 2,110,184 discloses a process for puffing a fruit as well as dehydrating a fruit. In the disclosed process after an initial drying period, and during a second drying period, the vacuum chamber is subjected to a momentary increase in pressure followed by a sudden reduction in pressure and restoration of the former drying conditions. The result is that the fruit being dried will be puffed as a result of the moisture that is vaporized during the rapid reduction of the pressure. The escaping vaporous moisture puffs the fruit. After the puffed fruit has reached the desired low moisture level the temperature is reduced and air is then introduced into the vacuum chamber. Both procedures are conducted in order not to collapse the fragile porous structure of the puffed fruit.
U.S. Pat. No. 2,283,302 discloses the method of vacuum frying foods whereby a fluid is used in the vacuum chamber in order to apply heat to the foods that are undergoing drying. The objective in this patent is to evenly apply heat to the foods being dehydrated. In this process the food being dehydrated is immersed in an edible fat, ethyl alcohol, glycerin or other medium. It is possible to puff the food being dried in this process by raising the temperature of the liquid medium and to thereby cause an increase in the rate of the evaporation of moisture from the food.
U.S. Pat. No. 2,587,939 is directed to a vacuum frying process similar to that of U.S. Pat. No. 2,283,302. However, in this process it is disclosed that in a final step the dehydrated food can be restored to atmospheric pressure in the presence of the liquid used as the medium to heat the food or in the presence of a liquid exchanged for that used during drying. The result is a food product wherein the puffed structure is impregnated with a liquid.
U.S. Pat. No. 2,712,698 is related to the vacuum frying processes of the foregoing two patents and is directed to a method of removing the retained liquid heating medium from the pores of the dried fruit. That is, even when there is an attempt to remove all of the liquid in which the food has been vacuum dried, there will remain residues of the heated liquid in the dried food product. This is preferably removed by using low vapor pressure hydrocarbons, ethers, and halocarbons to extract the remaining liquid heating medium. The objective is to produce a purer product.
The problem of fruit and vegetable discoloration has also been addressed. In U.S. Pat. No. 1,842,720 there is disclosed a technique of treating fruits and vegetables with a solution containing sulfurous acid and/or a salt that will yield sulfurous acid. The solution should also contain some hydrogen peroxide which will react with sodium acid sulfite to change it to sulfate.
In U.S. Pat. No. 3,305,366 there is disclosed a process for preserving the color of fresh fruits by contacting the fruits with a solution containing pyrophosphoric acid and an alkali sulfite or bisulfite. This method for preserving fruits is shown to be superior to treatment with solutions containing ascorbic acid or citric acid.
U.S. Pat. No. 3,754,938 discloses a technique for preserving apple slices using a solution consisting of ascorbic acid, calcium chloride and sodium bicarbonate. The sodium bicarbonate is added to maintain a pH of 7 to 9 during treatment. When this treating solution is used a sulfiting treatment does not have to be used.
U.S. Pat. No. 3,894,157 discloses a method for stabilizing the color of freeze-dried carrots using ascorbic acid or erythrobic acid. These acids are noted to be antioxidants. An aqueous solution containing one of these acids is sprayed onto the carrots. When ascorbic acid is the acid, the ascorbic acid is in a concentration of about 1 to 3 percent of the solution. The carrots are then freeze-dried.
U.S. Pat. No. 3,987,208 discloses a process for extending the storage life of cut lettuce. This consists of treating the lettuce leaves with an aqueous solution of sodium meta bisulfite, citric acid, ascorbic acid, and potassium sorbate. Any excess solution is drained from the lettuce. The lettuce is then placed in bags that have a low gaseous transmission rate. This treatment increases the shelf life of the lettuce.
U.S. Pat. No. 4,011,348 discloses the treatment of raw fruits and vegetables with an aqueous solution having a pH between about 4 and 7.5 and containing a selected ascorbate ion concentration. The ascorbate is sodium ascorbate and ascorbic acid. However, it is directed that ascorbic acid should not be used alone. It results in an excessively low pH.
U.S. Pat. No. 4,504,504 discloses a technique for preserving the texture of diced fresh foods through the use of gelled polyuronic acids. This is stated to be superior to the use of citric acid or calcium chloride in preserving dried fresh food products.
U.S. Pat. No. 4,650,686 discloses a method for reducing excessive browning in a baked good containing reducing sugars. This is accomplished by adding to the dough baking soda and fatty acid monoglyceride esters of polycarboxylic acids. The agents minimize reducing sugar browning during the baking process.
In addition to these patents an article titled "Controlling Storage and Processing Conditions Helps Produce Light Colored Chips From Sweet Potatoes" from Food Product Development, May 1977, discloses that color development in sweet potatoes being formed into sweet potato chips is also an ongoing problem. The recognized problem is that potatoes, such as the Irish sweet potato, will undergo a darkening discoloration while being processed into potato chips. The result would be a darkened potato chip after frying. The conclusions reached in this article are that an acceptably colored chip can be produced from potatoes stored for 48 hours at 23.degree. C. and then dipped in a solution containing 100 ppm sulfur dioxide. The dipped potato chips are then fried. If a lighter colored fried potato chip is desired the strength of the sulfur dioxide solution should be increased. Although a dip in a sulfur dioxide containing solution is preferred a water dip also has an influence on chip color.
Sulfur dioxide is a known preservative for various fruits and vegetables. Sulfur dioxide will also prevent a product from discoloring. It is used as a fruit or vegetable pretreatment in many vacuum frying processes. However, it is an additive to which some people are allergic. As a result, the use of sulfur dioxide to preserve food products is decreasing.
With regard to tenderizing fruits and vegetables U.S. Pat. No. 3,535,126 discloses a method of increasing the permeability of cellular foodstuff materials. The method consists of subjecting the foodstuff to a pressure of greater than about 50 p.s.i.g. while freezing the foodstuff. The pressure is released after the foodstuff is frozen. The resulting product is more permeable and yet the cell walls are still intact.
U.S. Pat. No. 3,792,184 discloses a technique for removing precipitating substances from plant extracts by a freezing and thawing process. Plant extracts such as extracts from cocoa powder or cinnamon powder contain undesirable precipitating substances such as proteins. These can be removed by freezing the extract and then raising the temperature of the extract to thaw the extract. Any precipitates are then easily removed by filtering or centrifuging the extract. Apparently this freeze-thaw processing coagulates these precipitating substances.
U.S. Pat. No. 4,495,205 discloses a process for making a meat analog product. In the process pasteurized vegetable protein is heated in an aqueous medium to remove volatiles. The heating is followed by a freezing step wherein the structure of the texturized vegetable protein is disrupted and its water binding capacity decreased. The frozen texturized vegetable product is then thawed and combined with a binder. This is then formed into a loaf and frozen. Freezing aids in activating the binder. The frozen product is then thawed and cooked.
In addition to these patents which discuss the freezing and/or thawing of substances, the book "The Freezing Preservation of Foods" by Donald K. Tressler and Clifford F. Evers, The Avi Publishing Co. New York 1947 discusses on pages 256 to 259 the changes caused in foods by freezing. Included is a discussion of the effect on the cell walls of foods from a freezing and thawing cycle.